Systems and methods for photometric normalization in array cameras

ABSTRACT

Systems and methods for performing photometric normalization in an array camera in accordance with embodiments of this invention are disclosed. The image data of scene from a reference imaging component and alternate imaging components is received. The image data from each of the alternate imaging components is then translated to so that pixel information in the image data of each alternate imaging component corresponds to pixel information in the image data of the reference component. The shifted image data of each alternate imaging component is compared to the image data of the reference imaging component to determine gain and offset parameters for each alternate imaging component. The gain and offset parameters of each alternate imaging component is then applied to the image data of the associate imaging to generate corrected image data for each of the alternate imaging components.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. application Ser. No.14/213,697, filed Mar. 14, 2014, which application claims priority toU.S. Provisional Patent Application No. 61/785,797, filed Mar. 14, 2013,the disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to digital cameras and morespecifically to systems and methods for evaluating imaging conditions.

BACKGROUND

The quality of an image captured by a digital camera can be influencedby factors including the exposure and focal plane settings of the cameraand the dynamic range within a scene. The exposure (duration of timewhich light is sampled by pixels in an image sensor) impacts the colorshades and tone and the focal plane settings impact a captured image'ssharpness.

The dynamic range within a scene is the difference in brightness fromthe darkest to brightest sections of the scene. Likewise, the dynamicrange of an image sensor is the difference in brightness from thedarkest to brightest sections that the image sensor is able to capture.Depending on the dynamic range within a particular scene, the maximumdynamic range of an image sensor can be many times smaller than thescene's dynamic range. Thus, digital cameras may not be able toaccurately capture the full range of brightness in any given scene.Various techniques including auto-exposure, autofocus and high dynamicrange imaging have been developed to improve the quality of imagescaptured using digital cameras.

In many image capture devices the sensitivity of the device to lightintensity can be adjusted by manipulating pixel integration time, pixelgain, and/or iris/lens aperture. Further, metering and auto-exposurealgorithms can be used to optimize the above parameters (some of theseparameters may be specified or fixed). Auto-exposure algorithms utilizemethods to capture images at optimal mean brightness levels by adjustingthe exposure time (or focal plane settings). Such algorithms generallyperform an iterative process that captures an image at a known exposuretime and based on the characteristics of the captured image, sets theexposure time (or focal plane settings) to capture following images atmore optimal mean brightness levels.

Most surfaces reflect incident light with some amount of scattering.Thus, the light intercepted by a camera is roughly isotropic with asmall region around the vantage point of the camera. Thus, individualimaging components of an array camera should ideally provide the samenumerical representation of an object in the individual images capturedby each of the imaging components. However, non-idealities exist in anarray camera and its individual imaging components due to manufacturingtolerances and other aberrations.

As such, the numerical representation for the same point in space ascaptured in the image data of each individual imaging component maydiffer. The differences may be subtle such as those differences causedby among other things, the differences in focal length, aperture ratios,and image sensor sensitivity in the individual imaging components. Someof these differences can be treated as constants and may be accountedfor by correction factors determined through a calibration process.

However, there are some differences that are introduced by the scenebeing imaged that cannot be compensated for ahead of time by correctionfactors. One example is veiling glare. Veiling glare occurs when theimage projected onto the pixels or sensors of an imaging component by alens system includes the intended image and an erroneous internallyscattered set of photons. The internally scattered set of photons mayoriginate from anywhere in front of the imaging component including bothwithin and outside the Field of View (FoV) of the imaging component.This causes the image projected onto the pixels or sensors of theimaging component at a given point to have more than or less than theexpected photons. Additional non-idealities may also exist including,but not limited to, contaminants on a protective window over the arraycamera installed in a device. The contaminants may change thephoto-response function for each of the individual imaging components byscattering or absorbing some of the photons entering the optical system.

It is a problem if the individual imaging components of the array camerado not report the same value for a given point in scene space in theirimage data. If the values for the same point in space differ in theimage data of individual imaging components, the parallax detectionbetween the different images may fail or become erroneous. Also, a noisesignal may be introduced into fused images from the local differences inthe numerical values of the image data from different imagingcomponents.

SUMMARY OF THE INVENTION

The above and other problems are solved an advance in the art is made bysystems and methods for providing photometric normalization for an arraycamera in accordance with embodiments of this invention. In accordancewith embodiments of this invention, one or more of the imagingcomponents of the array camera are designated as a reference imagingcomponent and each of the remaining imaging components in the arraycamera is an alternate imaging component. Each of the alternate imagingcomponents is associated with at least one of the reference imagingcomponents. In accordance with embodiments of this invention, aphotometric normalization process is performed after a scene has beencaptured by the array camera generating image data from each of theindividual imaging components of the array camera.

The following process is performed for each reference imaging componentand the alternate imaging components associated with each of thereference imaging components in accordance with embodiments of thisinvention. A nominal parallax shift is determined to translate pixelinformation in the imaging data of each alternate imaging component tocorresponding pixel information in the imaging data of the associatedreference imaging component. A low pass filter is then applied to imagedata of the reference camera and each of the associated translatedimaging components. For each associate imaging device, the pixelinformation from the translated and low-pass filtered image data of theassociate imaging device is compared to the corresponding pixelinformation of the low-pass filtered reference image to compute a gainand offset parameter transformation, which, when applied to thealternate images will photometrically match the two images, therebyreducing or eliminating the photometric imbalance among the images inthe array. The computed gain and offset parameters may then applied tothe image data of the associate imaging device to photometricallynormalize the image data with respect to the reference imaging device.

One embodiment of the method of the invention includes: receiving imagedata for a scene captured by the reference imaging component; receivingimage data for a scene captured by each of plurality of alternateimaging components; determining a nominal parallax for image data ofeach of the plurality of alternate imaging components that translateinformation for a particular pixel in the image data of a particularalternate imaging component to a corresponding pixel in the referenceimaging component; applying the nominal parallax of each particularalternate imaging component to the image data of the particularalternate imaging component; applying a low pass filter to the imagedata from the reference imaging component and the shifted image data ofeach particular alternate imaging component; and computing gain andoffset parameters for each particular alternate imaging components fromthe low pass filtered shifted image data of the particular alternateimaging component and the low pass filtered image data of the referenceimaging component.

A further embodiment also includes applying the gain and offsetparameters of each particular alternate imaging component to the imagedata captured by the particular alternate imaging component to formphotometrically normalized image data for each particular alternateimaging component.

Another embodiment also includes determining regions of high contrast inthe low pass filtered shifted image data of each particular alternateimaging component.

A still further embodiment includes storing determined the regions ofhigh contrast in the low pass filtered image data of each particularalternate imaging component for further correction processing.

In still another embodiment, the computing of the gain and offsetparameters is performed on a pixel by pixel basis for the image data ofeach of the plurality of alternate imaging components.

In a yet further embodiment, the computing of the gain and offsetparameters is performed on regions of pixels for the image data of eachof the plurality of alternate imaging components.

Yet another embodiment also includes: comparing each gain parameter andeach offset parameter for each of the plurality of alternate imagingcomponent to a threshold value; and setting each gain parameter and eachoffset parameter determined to at least meet the threshold value to apredetermined value.

An embodiment of a system of the invention includes: an array cameraincluding a plurality of imaging components that capture image data of ascene including a reference imaging component and plurality of alternateimaging components; a memory; and a processor that is configured byinstructions stored in the memory to: receive image data for a scenecaptured by the reference imaging component, receive image data for ascene captured by each of plurality of alternate imaging components,determine a nominal parallax for image data of each of the plurality ofalternate imaging components that translate information for a particularpixel in the image data of a particular alternate imaging component to acorresponding pixel in the reference imaging component, apply thenominal parallax of each particular alternate imaging component to theimage data of the particular alternate imaging component, apply a lowpass filter to the image data from the reference imaging component andthe shifted image data of each particular alternate imaging component,and compute gain and offset parameters for each particular alternateimaging components from the low pass filtered shifted image data of theparticular alternate imaging component and the low pass filtered imagedata of the reference imaging component.

In a further embodiment, the processor is further configured by theinstructions to apply the gain and offset parameters of each particularalternate imaging component to the image data captured by the particularalternate imaging component to form photometrically normalized imagedata for each particular alternate imaging component.

In another embodiment, the processor is further configured by theinstructions to determine regions of high contrast in the low passfiltered shifted image data of each particular alternate imagingcomponent.

In a still further embodiment, the processor is further configured bythe instructions to store the determined regions of high contrast in thelow pass filtered image data of each particular alternate imagingcomponent for further correction processing.

In still another embodiment, the computing of the gain and offsetparameters is performed on a pixel by pixel basis for the image data ofeach of the plurality of alternate imaging components.

In a yet further embodiment, the computing of the gain and offsetparameters is performed on regions of pixels for the image data of eachof the plurality of alternate imaging components.

In yet another embodiment, the processor is further configured by theinstructions to: compare each gain parameter and each offset parameterfor each of the plurality of alternate imaging component to a thresholdvalue; and set each gain parameter and each offset parameter determinedto at least meet the threshold value to a predetermined value.

Another further embodiment of the invention includes: receiving imagedata for a scene captured by the reference imaging component; receivingimage data for a scene captured by each of plurality of alternateimaging components; determining a nominal parallax for image data ofeach of the plurality of alternate imaging components that translateinformation for a particular pixel in the image data of a particularalternate imaging component to a corresponding pixel in the referenceimaging component; applying the nominal parallax of each particularalternate imaging component to the image data of the particularalternate imaging component; applying a low pass filter to the imagedata from the reference imaging component and the shifted image data ofeach particular alternate imaging component; and computing gain andoffset parameters for each particular alternate imaging components fromthe low pass filtered shifted image data of the particular alternateimaging component and the low pass filtered image data of the referenceimaging component.

In still another further embodiment, the method further comprisesapplying the gain and offset parameters of each particular alternateimaging component to the image data captured by the particular alternateimaging component to form photometrically normalized image data for eachparticular alternate imaging component.

In yet another further embodiment, the method further comprisesdetermining regions of high contrast in the low pass filtered shiftedimage data of each particular alternate imaging component.

In another further embodiment again, the method further comprisesstoring determined the regions of high contrast in the low pass filteredimage data of each particular alternate imaging component for furthercorrection processing.

In another further additional embodiment, the computing of the gain andoffset parameters is performed on a pixel by pixel basis for the imagedata of each of the plurality of alternate imaging components.

In still yet another further embodiment, the computing of the gain andoffset parameters is performed on regions of pixels for the image dataof each of the plurality of alternate imaging components.

In still another further embodiment again, the method further comprises:comparing each gain parameter and each offset parameter for each of theplurality of alternate imaging components to a threshold value; andsetting each gain parameter and each offset parameter determined to atleast meet the threshold value to a predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an array camera in accordance with anembodiment of the invention.

FIG. 2 conceptually illustrates an optic array and an imager array in anarray camera module in accordance with an embodiment of the invention.

FIG. 3 is an architecture diagram of an imager array in accordance withan embodiment of the invention.

FIG. 4 is a high level circuit diagram of pixel control and readoutcircuitry for a plurality of focal planes in an imager array inaccordance with an embodiment of the invention.

FIG. 5 conceptually illustrates a layout of color filters and thelocation of a reference imaging component and an alternate imagingcomponent in an array camera module in accordance with an embodiment ofthe invention.

FIG. 6 is a flow chart illustrating a process for performing photometricnormalization for an array camera in accordance with embodiments of theinvention.

DETAILED DISCLOSURE OF THE INVENTION

Turning now to the drawings, systems and methods for measuring sceneinformation while capturing images using array cameras in accordancewith embodiments of the invention are illustrated. Array camerasincluding camera modules that can be utilized to capture image data fromdifferent viewpoints (i.e. light field images) are disclosed in U.S.patent application Ser. No. 12/935,504 entitled “Capturing andProcessing of Images using Monolithic Camera Array with HeterogeneousImagers” to Venkataraman et al. In many instances, fusion and superresolution processes such as those described in U.S. patent applicationSer. No. 12/967,807 entitled “Systems and Methods for Synthesizing HighResolution Images Using Super-Resolution Processes” to Lelescu et al.,can be utilized to synthesize a higher resolution 2D image or a stereopair of higher resolution 2D images from the lower resolution images inthe light field captured by an array camera. The terms high or higherresolution and low or lower resolution are used here in a relative senseand not to indicate the specific resolutions of the images captured bythe array camera. The disclosures of U.S. patent application Ser. No.12/935,504 and U.S. patent application Ser. No. 12/967,807 are herebyincorporated by reference in their entirety.

In accordance with embodiments of this invention, a photometricnormalization is performed on image data captured by an array camera.The photometric normalization is performed to determine local offset andgain coefficients for the image data from alternate imaging componentswith respect to a reference imaging component. The gain and offsetcoefficients correct the image data of the alternate imaging componentto account for differences introduced by the scene being imaged. Inparticular, the gain coefficient corrects for the resultant attenuationof photons caused by veiling glare and other scene related issues andthe offset coefficient corrects for the resultant spurious or additionalphotons introduced by veiling glare or other scene independent issues.Systems and methods for performing photometric normalization of imagedata captured by an array camera in accordance with embodiments of theinvention are discussed further below.

Array Cameras

Array cameras in accordance with embodiments of the invention caninclude a camera module and a processor. An array camera in accordancewith an embodiment of the invention is illustrated in FIG. 1. The arraycamera 100 includes a camera module 102 with an array of individualimaging components 104 where an array of individual imaging componentsrefers to a plurality of imaging components in a particular arrangement,such as (but not limited to) the square arrangement utilized in theillustrated embodiment. The camera module 102 is connected to theprocessor 106 and the processor 106 is connected to a memory 108.Although a specific array camera is illustrated in FIG. 1, any of avariety of different array camera configurations can be utilized inaccordance with many different embodiments of the invention.

Array Camera Modules

Camera modules in accordance with embodiments of the invention can beconstructed from an imager array and an optic array. A camera module inaccordance with an embodiment of the invention is illustrated in FIG. 2.The camera module 200 includes an imager array 230 including an array offocal planes 240 along with a corresponding optic array 210 including anarray of lens stacks 220. Within the array of lens stacks, each lensstack 220 creates an optical channel that forms an image of the scene onan array of light sensitive pixels within a corresponding focal plane240. Each pairing of a lens stack 220 and focal plane 240 forms a singlecamera 104 within the camera module. Each pixel within a focal plane 240of a camera 104 generates image data that can be sent from the camera104 to the processor 108. In many embodiments, the lens stack withineach optical channel is configured so that pixels of each focal plane240 sample the same object space or region within the scene. In severalembodiments, the lens stacks are configured so that the pixels thatsample the same object space do so with sub-pixel offsets to providesampling diversity that can be utilized to recover increased resolutionthrough the use of super-resolution processes.

In several embodiments, color filters in individual imaging componentscan be used to pattern the camera module with it filter groups asfurther discussed in U.S. Provisional Patent Application No. 61/641,165entitled “Camera Modules Patterned with pi Filter Groups” filed May 1,2012, the disclosure of which is incorporated by reference herein in itsentirety. The use of a color filter pattern incorporating it filtergroups in a 4×4 array is illustrated in FIG. 5. These imaging componentscan be used to capture data with respect to different colors, or aspecific portion of the spectrum. In contrast to applying color filtersto the pixels of the individual imaging components, color filters inmany embodiments of the invention are included in the lens stack. Forexample, a green color imaging component can include a lens stack with agreen light filter that allows green light to pass through the opticalchannel. In many embodiments, the pixels in each focal plane are thesame and the light information captured by the pixels is differentiatedby the color filters in the corresponding lens stack for each filterplane. Although a specific construction of a camera module with an opticarray including color filters in the lens stacks is described above,camera modules including it filter groups can be implemented in avariety of ways including (but not limited to) by applying color filtersto the pixels of the focal planes of the camera module similar to themanner in which color filters are applied to the pixels of aconventional color camera. In several embodiments, at least one of theimaging components in the camera module can include uniform colorfilters applied to the pixels in its focal plane. In many embodiments, aBayer filter pattern is applied to the pixels of one of the imagingcomponents in a camera module. In a number of embodiments, cameramodules are constructed in which color filters are utilized in both thelens stacks and on the pixels of the imager array.

In several embodiments, an array camera generates image data frommultiple focal planes and uses a processor to synthesize one or moreimages of a scene. In certain embodiments, the image data captured by asingle focal plane in the sensor array can constitute a low resolutionimage (the term low resolution here is used only to contrast with higherresolution images), which the processor can use in combination withother low resolution image data captured by the camera module toconstruct a higher resolution image through Super Resolution processing.

Although specific array cameras are discussed above, many differentarray cameras are capable of utilizing π filter groups in accordancewith embodiments of the invention. Imager arrays in accordance withembodiments of the invention are discussed further below.

Imager Arrays

An imager array in which the image capture settings of a plurality offocal planes or imaging components can be independently configured inaccordance with an embodiment of the invention is illustrated in FIG. 3.The imager array 300 includes a focal plane array core 302 that includesan array of focal planes 304 and all analog signal processing, pixellevel control logic, signaling, and analog-to-digital conversion (ADC)circuitry. The imager array also includes focal plane timing and controlcircuitry 306 that is responsible for controlling the capture of imageinformation using the pixels. In a number of embodiments, the focalplane timing and control circuitry utilizes reset and read-out signalsto control the integration time of the pixels. In other embodiments, anyof a variety of techniques can be utilized to control integration timeof pixels and/or to capture image information using pixels. In manyembodiments, the focal plane timing and control circuitry 306 providesflexibility of image information capture control, which enables featuresincluding (but not limited to) high dynamic range imaging, high speedvideo, and electronic image stabilization. In various embodiments, theimager array includes power management and bias generation circuitry308. The power management and bias generation circuitry 308 providescurrent and voltage references to analog circuitry such as the referencevoltages against which an ADC would measure the signal to be convertedagainst. In many embodiments, the power management and bias circuitryalso includes logic that turns off the current/voltage references tocertain circuits when they are not in use for power saving reasons. Inseveral embodiments, the imager array includes dark current and fixedpattern (FPN) correction circuitry 310 that increases the consistency ofthe black level of the image data captured by the imager array and canreduce the appearance of row temporal noise and column fixed patternnoise. In several embodiments, each focal plane includes referencepixels for the purpose of calibrating the dark current and FPN of thefocal plane and the control circuitry can keep the reference pixelsactive when the rest of the pixels of the focal plane are powered downin order to increase the speed with which the imager array can bepowered up by reducing the need for calibration of dark current and FPN.

In many embodiments, a single self-contained chip imager includes focalplane framing circuitry 312 that packages the data captured from thefocal planes into a container file and can prepare the captured imagedata for transmission. In several embodiments, the focal plane framingcircuitry includes information identifying the focal plane and/or groupof pixels from which the captured image data originated. In a number ofembodiments, the imager array also includes an interface fortransmission of captured image data to external devices. In theillustrated embodiment, the interface is a MIPI CSI 2 output interface(as specified by the non-profit MIPI Alliance, Inc.) supporting fourlanes that can support read-out of video at 30 fps from the imager arrayand incorporating data output interface circuitry 318, interface controlcircuitry 316 and interface input circuitry 314. Typically, thebandwidth of each lane is optimized for the total number of pixels inthe imager array and the desired frame rate. The use of variousinterfaces including the MIPI CSI 2 interface to transmit image datacaptured by an array of imagers within an imager array to an externaldevice in accordance with embodiments of the invention is described inU.S. Pat. No. 8,305,456, entitled “Systems and Methods for TransmittingArray Camera Data”, issued Nov. 6, 2012, the disclosure of which isincorporated by reference herein in its entirety.

Although specific components of an imager array architecture arediscussed above with respect to FIG. 3, any of a variety of imagerarrays can be constructed in accordance with embodiments of theinvention that enable the capture of images of a scene at a plurality offocal planes in accordance with embodiments of the invention.Independent focal plane control that can be included in imager arrays inaccordance with embodiments of the invention are discussed furtherbelow.

Independent Focal Plane Control

Imager arrays in accordance with embodiments of the invention caninclude an array of focal planes or imaging components that canindependently be controlled. In this way, the image capture settings foreach focal plane in an imager array can be configured differently. As isdiscussed further below, the ability to configure active focal planesusing difference image capture settings can enable different cameraswithin an array camera to make independent measurements of sceneinformation that can be combined for use in determining image capturesettings for use more generally within the camera array.

An imager array including independent control of image capture settingsand independent control of pixel readout in an array of focal planes inaccordance with an embodiment of the invention is illustrated in FIG. 4.The imager array 400 includes a plurality of focal planes or pixelsub-arrays 402. Control circuitry 403, 404 provides independent controlof the exposure timing and amplification gain applied to the individualpixels within each focal plane. Each focal plane 402 includesindependent row timing circuitry 406, 408, and independent columnreadout circuitry 410, 412. In operation, the control circuitry 403, 404determines the image capture settings of the pixels in each of theactive focal planes 402. The row timing circuitry 406, 408 and thecolumn readout circuitry 410, 412 are responsible for reading out imagedata from each of the pixels in the active focal planes. The image dataread from the focal planes is then formatted for output using an outputand control interface 416.

Although specific imager array configurations are discussed above withreference to FIG. 4, any of a variety of imager array configurationsincluding independent and/or related focal plane control can be utilizedin accordance with embodiments of the invention including those outlinedin U.S. patent application Ser. No. 13/106,797, entitled “Architecturesfor Imager Arrays and Array Cameras”, filed May 12, 2011, the disclosureof which is incorporated by reference herein in its entirety. The use ofindependent focal plane control to capture image data using arraycameras is discussed further below.

Photometric Normalization for an Array Camera

In accordance with many embodiments of this invention, a photometricnormalization is performed on image data captured by an array camera.The photometric normalization is performed to determine local offset andgain coefficients for the image data from alternate imaging componentswith respect to a reference imaging component. The gain and offsetcoefficients correct the image data of the alternate imaging componentto account for differences introduced by the scene being imaged. Inparticular, the gain coefficient corrects for the resultant attenuationof photons caused by veiling glare and other scene related issues andthe offset coefficient corrects for the resultant spurious or additionalphotons introduced by veiling glare or other scene independent issues.

The normalization performed is based on the fact the one of theproperties of the veiling glare phenomenon and other scene relatederrors is that its effect on the photo-response of each of theindividual imaging components is typically low in spatial frequency.Thus, the photo-response of the imaging component does not changerapidly within an image area. Instead, the photo-response is relativelyslow changing. As the scene related errors may cause the image projectedon the imaging components to include either more or less photons thanpredicted by a flat-field calibration, some areas of the image of theindividual image sensor may be brighter or darker versus the image fromother imaging components.

These scene related errors may be corrected for or normalized out bycomputing the above described gain and offset coefficients. Thesecoefficients can be determined because the response of an imagingcomponent in the raw domain is designed to be linear. Thus, the typicaly=mx+c formula may be used to define the response in the followingmanner:y _(i,j) =m _(i,j) x _(i,j) +c  (1)

Where:

y_(i,j)=numerical output value of the sensor for a given position in theimage;

x_(i,j)=photon input to the sensor at a given position;

m_(i,j)=conversion gain of the sensor at a given position as determinedby calibration; and

c=pedestal black level of the sensor.

Scene related errors can be thought of as resulting in the followingmodification to the formula:y _(i,j) =m _(i,j) x _(i,j)(Gvg _(i,j))+C+(Ovg _(i,j))  (2)

Where:

Gvg is the gain coefficient representing the resultant attenuation ofphotons; and

Ovg is the offset coefficient representing the resultant spurious oradditional photons.

To normalize the image data from alternate imaging components withrespect to the image data of a reference imaging component, the gain andoffset coefficients for the alternate imaging components with respect tothe reference imaging component can be computed and applied to theimaging data of the alternate imaging component to negate their effects.The use of a color filter pattern incorporating π filter groups in a 4×4array is illustrated in FIG. 5. In the array camera 500, a first imagingcomponent 504 configured to capture green light can be selected as areference imaging component and a second imaging component 506configured to capture green light can be selected as an alternateimaging component. As can readily be appreciated, any pair of camerasconfigured to capture the same frequency of light can be selected as areference imaging component and an associate imaging component. Aprocess for performing this photometric normalization in accordance withembodiments of this invention is illustrated in FIG. 6.

Process 600 includes obtaining the image data for a scene from areference imaging component and the alternate imaging componentsassociated with the reference imaging component (605). This may be doneby capturing an image of the scene with an array camera causing thereference and alternate imaging components to each generate image dataof the scene. Alternatively, the image data may have been previouslycaptured and is read from a memory.

If the array camera includes more than one reference imaging component,a reference imaging component is selected to perform the normalization(605). A low pass filter is then applied to the image data of thereference imaging component (610). The low pass filter removes any highfrequency components in the reference image data.

The following process is then performed to normalize the image data fromeach of the alternate imaging components associated with referenceimaging component. An alternate imaging component is selected (615) andthe image data for the alternate imaging component is retrieved. Anominal parallax between the selected alternate imaging component andthe reference imaging component is determined (620). The nominalparallax may be read from memory if it was previously stored or may becomputed at the time of use.

In accordance with some embodiments of this invention, the nominalparallax may be determined by metering a region-of-interest within thefield of view and performing a coarse parallax estimate to determine aparallax shift that satisfies the metered region-of-interest. Inaccordance with some other embodiments, a nominal parallax shiftcorresponding to typical shooting distances may be used. In accordancewith still other embodiments, the depth map from a previously fullycomputed frame may be used to specify the nominal parallax shift.

The nominal parallax shift is then applied to the image data of thealternate camera to translate the pixel information in the image data tocorrespond with corresponding pixel information in the image data of thereference imaging component (625). A low pass filter is then applied tothe shifted image data of the alternate imaging component (630). Theshifted, low passed filtered image data of the alternate image componentis aligned with the low passed filtered image data from the referenceimaging component in a “strong” blurred alignment. A “strong” blurredalignment is when the images are aligned on surviving high-gradientedges in low frequency such that the images appear be aligned even ifthere some spatial misalignment due to error in the alignmentinformation. Errors in alignment information may be due to many factors,including, but not limited tom, taking the parallax at an incorrectfixed distance.

In accordance with some embodiments, high contrast components in theimage data from the alternate imaging component may be detected. Thehigh contrast components in the shifted image data are typically inareas where alignment errors caused by using the nominal parallax shiftbetween the shifted image data from the alternate imaging component andthe image data from the reference imaging component are apparent. Thesehigh-contrast edges may still cause differences even after the low passfilter is applied. Thus, these high contrast components are optionallydetected and stored as a data set, map, or other data structure (635).As these components have a greater probability of being erroneous evenafter the subsequent correction values are applied, the data set or mapmay be used to indicate components of the shifted image data from thealternate imaging component where later correction processes can beapplied modulate the corrected data if needed and/or desired.

The low pass filtered shifted image data of the alternate imagingcomponent is then compared to the low pass filtered image data of thereference imaging component to compute the gain and offset parametersfor the image data from the alternate imaging component (640). The lowpassed filtered shifted image data is used to determine the gain andoffset parameter because most photometric imbalances occur in lowfrequency. Thus, the gain and offset parameters to locally correct thephotometric imbalance determined using the low pass filtered image datawill correct photometric imbalance in the original image data as thephotometric imbalance if in the lower frequency.

In accordance with some embodiments, the gain and offset parameters arecalculated on a per pixel basis. In accordance with these embodiments,the gain and offset parameters are calculated based on a regionsurrounding each pixel. For example, a region of 9×9 pixels surroundinga pixel may be analyzed to determine the distribution of values withinthe region. In other embodiments, any of a variety of fixed or adaptiveregions can be utilized including regions that have different shapes indifferent regions of the image. A level of contrast exists in the regionwithin the image data of the alternate imaging component. The goal ofthe computation is to determine gain and offset parameters for the pixelin shifted image data that matches the value of the pixel data to thevalue of the pixel data of the reference imaging component. This may beachieved by comparing the mean and variance of the data for the pixelarea to the mean and variance of the data for a corresponding pixel areain the reference image data.

In accordance with some embodiments of this invention, the followingequation may be used to perform the comparisons and determine the gainand offset parameters:

${\hat{a} = \frac{\left\lbrack {\sum\limits_{i}{\sum\limits_{j}{{y\left( {i,j} \right)}{x\left( {i,j} \right)}}}} \right\rbrack - {N_{1}N_{2}\overset{\_}{y}\overset{\_}{x}}}{\left\lbrack {\sum\limits_{i}{\sum\limits_{j}{x^{2}\left( {i,j} \right)}}} \right\rbrack - {N_{1}N_{2}{\overset{\_}{x}}^{2}}}},{\hat{b} = {\overset{\_}{y} - {\hat{a}\overset{\_}{x}}}},{where}$${\overset{\_}{x} = {\frac{1}{N_{1}N_{2}}{\sum\limits_{i}{\sum\limits_{j}{x\left( {i,j} \right)}}}}},{\overset{\_}{y} = {\frac{1}{N_{1}N_{2}}{\sum\limits_{i}{\sum\limits_{j}{y\left( {i,j} \right)}}}}},$

Where:

x=the image to be corrected

y=the reference image

N₁, N₂=number of pixels horizontally and vertically of the analyzedregion around the pixel being computed.

i,j are indices into the images within the bounds defined by N₁ and N₂.

â=Gvg_(i,j), gain coefficient computed for a specific value of i and j.

{circumflex over (b)}=Ovg_(i,j), offset term computed for a specificvalue of i and j.

In accordance with some embodiments, limits may be applied to thecomputation such that values computed for the gain and offset parametersare constrained in some way. In accordance with some of theseembodiments, the gain and offset parameters may be prevented from beingtoo large by being compared to a threshold and being set to apredetermined value if the threshold is at least met.

In accordance with other embodiments, the gain and offset parameters maybe determined for regions of the associate image data instead of a perpixel basis by using a sparse grid. The subsequent spatially varyingvalues of the gain and offset parameters may be interpolated to yieldthe correct value for each pixel. One skilled in the art will recognizethat still other methods of determining the gain and offset parametersmay be used without departing from the embodiments of this invention.

The determined gain and offset parameters for each pixel are thenapplied to the corresponding information for each pixel in the originalimage data of the associate reference component (645). In accordancewith some embodiments, the map or data set of high contrast regions maybe used to determine regions where the calculations may be erroneous andadditional processes may need to be performed to normalize the data.

The process (615-645) for alternate imaging components associated theselected reference is then repeated until normalization is performed foreach alternate imaging component associated with the selected referenceimaging component (650). The process is likewise repeated for eachreference imaging component in the array camera (655).

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof. It istherefore to be understood that the present invention may be practicedotherwise than specifically described, without departing from the scopeand spirit of the present invention. Thus, embodiments of the presentinvention should be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A method performed by a processing system toprovide a photometric normalization in an array camera system having aplurality of imaging components, the method comprising: receiving imagedata for a scene captured by a first one of the plurality of imagingcomponents; receiving image data for a scene captured by a second one ofthe plurality of imaging components; determining a nominal parallax forimage data of the second one of the plurality the of imaging componentsthat translate information for a particular pixel in the image data ofthe second one of the plurality of imaging components to a correspondingpixel in the first one of the plurality of imaging components; applyingthe nominal parallax of the second one of the plurality of imagingcomponents to the image data of the second one of the plurality ofimaging components; applying a low pass filter to the image data fromthe first one of the plurality of imaging components and the shiftedimage data of the second one of the plurality of imaging components; andcomputing gain and offset parameters for the second one of the pluralityof imaging components from the low pass filtered shifted image data ofthe second one of the plurality of imaging components and the low passfiltered image data of the first one of the plurality of imagingcomponents.
 2. The method of claim 1 further comprising applying thegain and offset parameters of the second one of the plurality of imagingcomponents to the image data captured by the second one of the pluralityof imaging components to form photometrically normalized image data forthe second one of the plurality of imaging components.
 3. The method ofclaim 2 further comprising determining regions of high contrast in thelow pass filtered shifted image data of the second one of the pluralityof imaging components.
 4. The method of claim 3 further comprisingstoring the determined regions of high contrast in the low pass filteredimage data of the second one of the plurality of imaging components forfurther correction processing.
 5. The method of claim 1 wherein thecomputing of the gain and offset parameters is performed on a pixel bypixel basis for the image data of the second one of the plurality ofimaging components.
 6. The method of claim 1 wherein the computing ofthe gain and offset parameters is performed on regions of pixels for theimage data of the second one of the plurality of imaging components. 7.The method of claim 1 further comprising: comparing each gain parameterand each offset parameter for the second one of the plurality of imagingcomponents to a threshold value; and setting each gain parameter andeach offset parameter determined to at least meet the threshold value toa predetermined value.
 8. A system for providing a photometricnormalization in an array camera system having a plurality of imagingcomponents comprising: an array camera system including a plurality ofimaging components that each capture image data of a scene; a memory;and a processor that is configured by instructions stored in the memoryto: receive image data for a scene captured by a first one of theplurality of imaging components, receive image data for a scene capturedby a second one of the plurality of imaging components, determine anominal parallax for image data of each of the plurality of alternateimaging components that translate information for a particular pixel inthe image data of a particular alternate imaging component to acorresponding pixel in the first one of the plurality of imagingcomponents, apply the nominal parallax of the second one of theplurality of imaging components to the image data of the second one ofthe plurality of imaging components, apply a low pass filter to theimage data from the first one of the plurality of imaging components andthe shifted image data of the second one of the plurality of imagingcomponents, and compute gain and offset parameters for the second one ofthe plurality of imaging components from the low pass filtered shiftedimage data of the second one of the plurality of imaging components andthe low pass filtered image data of the first one of the plurality ofimaging components.
 9. The system of claim 8 wherein the processor isfurther configured by the instructions to apply the gain and offsetparameters of the second one of the plurality of imaging components tothe image data captured by the second one of the plurality of imagingcomponents to form photometrically normalized image data for the secondone of the plurality of imaging components.
 10. The system of claim 8wherein the processor is further configured by the instructions todetermine regions of high contrast in the low pass filtered shiftedimage data of the second one of the plurality of imaging components. 11.The system of claim 10 wherein the processor is further configured bythe instructions to store the determined regions of high contrast in thelow pass filtered image data of the second one of the plurality ofimaging components for further correction processing.
 12. The system ofclaim 8 wherein the computing of the gain and offset parameters isperformed on a pixel by pixel basis for the image data of the second oneof the plurality of imaging components.
 13. The system of claim 8wherein the computing of the gain and offset parameters is performed onregions of pixels for the image data of the second one of the pluralityof imaging components.
 14. The system of claim 8 wherein the processoris further configured by the instructions to: compare each gainparameter and each offset parameter for the second one of the pluralityof imaging components to a threshold value; and set each gain parameterand each offset parameter determined to at least meet the thresholdvalue to a predetermined value.
 15. A non-transitory medium readable bya processor that stores instructions that when read by the processorconfigure the processor to perform the method comprising: receivingimage data for a scene captured by a first one of the plurality ofimaging components of an array camera system; receiving image data for ascene captured by a second one of plurality the of imaging components ofthe array camera system; determining a nominal parallax for image dataof the second one of the plurality of imaging components that translateinformation for a particular pixel in the image data of the second oneof the plurality of imaging components to a corresponding pixel in thefirst one of the plurality of imaging components; applying the nominalparallax of the second one of the plurality of imaging components to theimage data of the second one of the plurality of imaging components;applying a low pass filter to the image data from the first one of theplurality of imaging components and the shifted image data of the secondone of the plurality of imaging components; and computing gain andoffset parameters for the second one of the plurality of imagingcomponents from the low pass filtered shifted image data of the secondone of the plurality of imaging components and the low pass filteredimage data of the first one of the plurality of imaging components. 16.The non-transitory medium of claim 15 wherein the method furthercomprises applying the gain and offset parameters of the second one ofthe plurality of imaging components to the image data captured by thesecond one of the plurality of imaging components to formphotometrically normalized image data for the second one of theplurality of imaging components.
 17. The non-transitory medium of claim16 wherein the method further comprises determining regions of highcontrast in the low pass filtered shifted image data of the second oneof the plurality of imaging components.
 18. The non-transitory medium ofclaim 17 wherein the method further comprises storing the determinedregions of high contrast in the low pass filtered image data of thesecond one of the plurality of imaging components for further correctionprocessing.
 19. The non-transitory medium of claim 15 wherein thecomputing of the gain and offset parameters is performed on a pixel bypixel basis for the image data of the second one of the plurality ofimaging components.
 20. The non-transitory medium of claim 15 whereinthe computing of the gain and offset parameters is performed on regionsof pixels for the image data of the second one of the plurality ofimaging components.